Exposure apparatus and method utilizing isolated reaction frame

ABSTRACT

A guided stage mechanism suitable for supporting a reticle in a photolithography machine includes a stage movable in the X-Y directions on a base. Laterally surrounding the stage is a rectangular window frame guide which is driven in the X-axis direction on two fixed guides by means of motor coils on the window frame guide co-operating with magnetic tracks fixed on the base. The stage is driven inside the window frame guide in the Y-axis direction by motor coils located on the stage co-operating with magnetic tracks located on the window frame guide. Forces from the drive motors of both the window frame guide and the stage are transmitted through the center of gravity of the stage, thereby eliminating unwanted moments of inertia. Additionally, reaction forces caused by the drive motors are isolated from the projection lens and the alignment portions of the photolithography machine. This isolation is accomplished by providing a mechanical support for the stage independent of the support for its window frame guide. The window frame guide is a hinged structure capable of a slight yawing (rotational) motion due to hinged flexures which connect the window frame guide members.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to precision motion stages and morespecifically to a stage suitable for use in a photolithography machineand especially adapted for supporting a reticle.

[0003] 2. Description of the Prior Art

[0004] Photolithography is a well known field especially as applied tosemiconductor fabrication. In photolithography equipment a stage (an X-Ymotion device) supports the reticle (i.e., mask) and a second stagesupports the semiconductor wafer, i.e. the work piece being processed.Sometimes only a single stage is provided, for the wafer or the mask.

[0005] Such stages are essential for precision motion in the X-axis andY-axis directions and often some slight motion is provided foradjustments in the vertical (Z-axis) direction. A reticle stage istypically used where the reticle is being scanned in a scanning exposuresystem, to provide smooth and precise scanning motion in one lineardirection and insuring accurate, reticle to wafer alignment bycontrolling small displacement motion perpendicular to the scanningdirection and a small amount of “yaw” (rotation) in the X-Y plane. It isdesirable that such an X-Y stage be relatively simple and be fabricatedfrom commercially available components in order to reduce cost, whilemaintaining the desired amount of accuracy. Additionally, many prior artstages include a guide structure located directly under the stageitself. This is not a desirable in a reticle stage since it is essentialthat a light beam be directed through the reticle and through the stageitself to the underlying projection lens. Thus a stage is needed whichdoes not include any guides directly under the stage itself, since thestage itself must define a fairly large central passage for the lightbeam.

[0006] Additionally, many prior art stages do not drive the stagethrough its center of gravity which undesirably induces a twistingmotion in the stage, reducing the frequency response of the stage.Therefore there is a need for an improved stage and especially onesuitable for a reticle stage.

SUMMARY

[0007] A precision motion stage mechanism includes the stage itselfwhich moves in the X-Y plane on a flat base. The stage is laterallysurrounded by a “window frame” guide structure which includes fourmembers attached at or near their corners to form a rectangularstructure. The attachments are flexures which are a special type ofhinge allowing movement to permit slight distortion of the rectangle. Inone version these flexures are thin stainless steel strips attached inan “X” configuration, allowing the desired degree of hinge movementbetween any two adjacent connected window frame members.

[0008] The window frame guide structure moves on a base against twospaced-apart and parallel fixed guides in e.g. the X axis direction,being driven by motor coils mounted on two opposing members of thewindow frame cooperating with magnetic tracks fixed on the base.

[0009] The window frame in effect “follows” the movement of the stageand carries the magnetic tracks needed for movement of the stage in theY axis direction. (It is to be understood that references herein to theX and Y axes directions are merely illustrative and for purposes oforientation relative to the present drawings and are not to be construedas limiting.)

[0010] The stage movement in the direction perpendicular (the Y axisdirection) to the direction of movement of the window frame isaccomplished by the stage moving along the other two members of thewindow frame. The stage is driven relative to the window frame by motorcoils mounted on the stage and cooperating with magnetic tracks mountedin the two associated members of the window frame.

[0011] To minimize friction, the stage is supported on the base by airbearings or other fluid bearings mounted on the underside of the stage.Similarly fluid bearings support the window frame members on their fixedguides. Additionally, fluid bearings load the window frame membersagainst the fixed guides and load the stage against the window frame. Soas to allow slight yaw movement, these loading bearings are springmounted. The stage itself defines a central passage. The reticle restson a chuck mounted on the stage. Light from an illuminating sourcetypically located typically above the reticle passes to the centralpassage through the reticle and chuck to the underlying projection lens.

[0012] It is to be understood that the present stage, with suitablemodifications, is not restricted to supporting a reticle but also may beused as a wafer stage and is indeed not limited to photolithographyapplications but is generally suited to precision stages.

[0013] An additional aspect in accordance with the present invention isthat the reaction force of the stage and window frame drive motors isnot transmitted to the support frame of the photolithography apparatusprojection lens but is transmitted independently directly to the earth'ssurface by an independent supporting structure. Thus the reaction forcescaused by movement of the stage do not induce undesirable movement inthe projection lens or other elements of the photolithography machine.

[0014] This physically isolating the stage reaction forces from theprojection lens and associated structures prevents these reaction forcesfrom vibrating the projection lens and associated structures. Thesestructures include the interferometer system used to determine the exactlocation of the stage in the X-Y plane and the wafer stage. Thus thereticle stage mechanism support is spaced apart from and independentlysupported from the other elements of the photolithography machine andextends to the surface of the earth.

[0015] Advantageously, the reaction forces from operation of the fourmotor coils for moving both the stage and its window frame aretransmitted through the center of gravity of the stage, therebydesirably reducing unwanted moments of force (i.e., torque). Thecontroller controlling the power to the four drive motor coils takesinto consideration the relative position of the stage and the frame andproportions the driving force accordingly by a differential drivetechnique.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 shows a top view of the present window frame guided stage.

[0017]FIG. 2 shows a side view of the window frame guided stage andassociated structures.

[0018]FIGS. 3A and 3B show enlarged views of portions of the structureof FIG. 2.

[0019]FIG. 4 shows a top view of a photolithography apparatus includingthe present window frame guided stage.

[0020]FIG. 5 shows a side view of the photolithography apparatus of FIG.4.

[0021]FIGS. 6A and 6B show a flexure hinge structure as used e.g. in thepresent window frame guided stage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022]FIG. 1 shows a top view of a stage mechanism in accordance withthe present invention. See also copending commonly owned and inventedU.S. patent application, Ser. No. 08/221,375 entitled “Guideless Stagewith Isolated Reaction Stage” filed Apr. 1, 1994, original docket no.NPI0500 which is incorporated herein by reference and shows a relatedmethod of supporting elements of a stage mechanism so as to isolatereaction forces from the projection lens and other parts of aphotolithography apparatus.

[0023] The stage 10 is (in plan view) a rectangular structure of a rigidmaterial (e.g., steel, aluminum, or ceramic). Two interferometry mirrors14A and 14B located on stage 10 interact conventionally withrespectively laser beams 16A and 16B. Conventionally, laser beams 16Aare two pairs of laser beams and laser beams 16B are one pair of laserbeam, for three independent distance measurements. The underside ofstage 10 defines a relieved portion 22 (indicated by a dotted line, notbeing visible in the plane of the drawing). A reticle 24 is located onstage 10 and held by conventional reticle vacuum groove 26 formed in theupper surface of chuck plate 28. Stage 10 also defines a centralaperture 30 (passage) below the location of reticle 24. Central aperture30 allows the light (or other) beam which penetrates through reticle 24to enter the underlying projection lens, as described further below. (Itis to be understood that the reticle 24 itself is not a part of thestage mechanism.) Moreover if the present stage mechanism is to be usedfor other than a reticle stage, i.e. for supporting a wafer, aperture 30is not needed.

[0024] Stage 10 is supported on a conventional rectangular basestructure 32 of e.g. granite, steel, or aluminum, and having a smoothplanar upper surface. The left and right edges (in FIG. 1) of basestructure 32 are shown as dotted lines, being overlain by otherstructures (as described below) in this view. In operation, stage 10 isnot in direct physical contact with its base structure 32; instead,stage 10 is vertically supported by, in this example, conventionalbearings such as gas bearings. In one embodiment three air bearings 36A,36B and 36C are used which may be of a type commercially available.

[0025] In an alternative air bearing/vacuum structure, the vacuumportion is physically separated from and adjacent to the air bearingportion. It is to be understood that the vacuum and compressed air areprovided externally via tubing in a conventional cable bundle andinternal tubing distribution system (not shown in the drawings forsimplicity). In operation stage 10 thereby floats on the air bearings36A, 36B, 36C approximately 1 to 3 micrometers above the flat topsurface of base structure 32. It is to be understood that other types ofbearings (e.g. air bearing/magnetic combination type) may be usedalternatively.

[0026] Stage 10 is laterally surrounded by the “window frame guide”which is a four member rectangular structure. The four members as shownin FIG. 1 are (in the drawing) the top member 40A, the bottom member40B, the lefthand member 40C, and the righthand member 40D. The fourmembers 40A-40D are of any material having high specific stiffness(stiffness to density ratio) such as aluminum or a composite material.These tour members 40A-40D are attached together by hinge structureswhich allow non-rigid movement of the four members relative to oneanother in the X-Y plane and about the Z-axis as shown in the drawing,this movement also referred to as a “yaw” movement. The hinge isdescribed in detail below, each hinge 44A, 44B, 44C and 44D being e.g.one or more metal flexures allowing a slight flexing of the window frameguide structure.

[0027] The window frame guide structure moves in the X axis (to the leftand right in FIG. 1) supported on horizontal surfaces of fixed guides46A and 46B, and supported on vertical surfaces of fixed guides 64A,64B. (It is to be understood that each pair of fixed guides 46A, 64A and46B, 64B could be e.g. a single L-shaped fixed guide, or otherconfigurations of fixed guides may be used.) Mounted on window frameguide member 40A are two air bearings 50A and 50B that cause the member40A to ride on its supporting fixed guide member 46A. Similarly airbearings 52A and 52B are mounted on the member 40B, allowing member 40Bto ride on its supporting fixed guide member 46B. Air bearings 50A, 50B,52A, 52B are similar to air bearings 36A, etc.

[0028] The window frame guide is driven along the X axis on fixed guides46A and 46B, 64A and 64B by a conventional linear motor, which includesa coil 60A which is mounted on window frame guide member 40A. Motor coil60A moves in a magnetic track 62A which is located in (or along) fixedguide 64A. Similarly, motor coil 60B which is mounted on window frameguide member 40B moves in magnetic track 62B which is located in fixedguide 64B. The motor coil and track combinations are part no. LM-310from Trilogy Company of Webster Tex. These motors are also called“linear commutator motors”. The tracks 62A, 62B are each a number ofpermanent magnets fastened together. The electric wires which connect tothe motor coils are not shown but are conventional. Other types oflinear motors may be substituted. It is to be understood that thelocations of the motor coils and magnetic tracks for each motor could bereversed, so that for instance the magnetic tracks are located on stage10 and the corresponding motor coils on the window frame guide members,at a penalty of reduced performance.

[0029] Similarly, stage 10 moves along the Y axis in FIG. 1 by means ofmotor coils 68A and 68B mounted respectively on the left and right edgesof stage 10. Motor coil 68A moves in magnetic track 70A mounted inwindow frame guide member 40C. Motor coil 68B moves in magnetic track70B mounted in window frame guide member 40D.

[0030] Also shown in FIG. 1 are air bearings 72A, 72B and 72C. Airbearing 72A is located on window frame guide member 40A and minimizesfriction between window frame guide member 40A and its fixed guide 64A.Similarly two air bearings 72B and 72C on window frame guide member 40Bminimize its friction with the fixed guide 64B. The use of a single airbearing 72A at one end and two opposing air bearings 72B and 72C at theother end allows a certain amount of yaw (rotation in the X-Y planeabout the Z-axis) as well as limited motion along the Z-axis. In thiscase, typically air bearing 72A is gimbal mounted, or gimbal mountedwith the gimbal located on a flexure so as to allow a limited amount ofmisalignment between the member 40A and fixed guide 64A.

[0031] The use of the air bearing 72A opposing bearings 72B and 72Cprovides a loading effect to keep the window frame guide in its properrelationship to fixed guides 64A, 64B. Similarly, an air bearing 76Aloads opposing air bearings 76B and 76C, all mounted on side surfaces ofthe stage 10, in maintaining the proper location of stage 10 relative tothe opposing window frame guide members 40B and 40D. Again, in this caseone air bearing such as 76A is gimbal mounted to provide a limitedamount of misalignment, or gimbal mounted with the gimbal on a flexure(spring). Air bearings 72A, 72B, 72C and 76A, 76B, and 76C areconventional air bearings.

[0032] The outer structure 80 in FIG. 1 is the base support structurefor the fixed guides 46A, 46B, 64A, 64B and the window frame guidemembers 40A, . . . , 40D of the stage mechanism, but does not supportstage base structure 32. Thus the underlying support is partitioned sothe reaction force on base support structure 80 does not couple into thestage base structure 32. Base support structure 80 is supported by itsown support pillars or other conventional support elements (not shown inthis drawing) to the ground, i.e. the surface of the earth or the floorof a building. An example of a suitable support structure is disclosedin above-referenced U.S. patent application Ser. No. 08/221,375 at FIGS.1, 1B, 1C. This independent support structure for this portion of stagemechanism provides the above-described advantage of transmitting thereaction forces of the reticle stage mechanism drive motors away fromthe frame supporting the other elements of the photolithographyapparatus, especially away from the optical elements including theprojection lens and from the wafer stage, thereby minimizing vibrationforces on the projection lens due to reticle stage movement. This isfurther described below.

[0033] The drive forces for the stage mechanism are provided as close aspossible through the stage mechanism center of gravity. As can beunderstood, the center of gravity of the stage mechanism moves with thestage 10. Thus the stage 10 and the window frame guide combine to definea joint center of gravity. A first differential drive control (notshown) for motor coils 60A, 60B takes into account the location of thewindow frame guide to control the force exerted by each motor coil 60A,60B to keep the effective force applied at the center of gravity. Asecond conventional differential drive control (not shown) for motorcoils 68A, 68B takes into account the location of stage 10 to controlthe force exerted by each motor coil 68A, 68B to keep the effectiveforce applied at the center of gravity. It is to be understood thatsince stage 10 has a substantial range of movement, that thedifferential drive for the motor coils 60A, 60B has a wide differentialswing. In contrast, the window frame guide has a more limited range ofmovement, hence the differential drive for the motor coils 68A, 68B hasa much lesser differential swing, providing a trim effect.Advantageously, use of the window frame guide maintains the reactionforces generated by movement of the reticle stage mechanism in a singleplane, thus making easier to isolate these forces from other parts ofthe photolithography apparatus.

[0034]FIG. 2 shows a cross-sectional view through line 2-2 of FIG. 1.The structures shown in FIG. 2 which are also in FIG. 1 have identicalreference numbers and are not described herein. Also shown in FIG. 2 isthe illuminator 90 which is a conventional element shown here withoutdetail, and omitted from FIG. 1 for clarity. Also shown without detailin FIG. 2 is the upper portion of the projection lens (barrel) 92. It isto be understood that the lower portion of the projection lens and otherelements of the photolithography apparatus are not shown in FIG. 2, butare illustrated and described below.

[0035] The supporting structure 94 for the projection lens 92 is alsoshown in FIG. 2. As can be seen, structure 94 is separated at all pointsby a slight gap 96 from the base support structure 80 for the reticlestage mechanism. This gap 96 isolates vibrations caused by movement ofthe reticle stage mechanism from the projection lens 92 and its support94. As shown in FIG. 2, stage 10 is not in this embodiment a flatstructure but defines the underside relieved portion 22 to accommodatethe upper portion of lens 92. Magnetic track 70A is mounted on top ofthe window frame guide 40B and similarly magnetic track 70B is mountedon top of the opposite window frame guide member 40D.

[0036]FIGS. 3A and 3B are enlarged views of portions of FIG. 2, withidentical reference numbers; FIG. 3A is the left side of FIG. 2 and FIG.3B is the right side of FIG. 2. Shown in FIG. 3A is the spring mounting78 for air bearing 76A. Air bearing 78A being spring mounted to a sidesurface of stage 10, this allows a certain amount of yaw (rotation inthe X-Y plane about the Z-axis) as well as limited motion along theZ-axis. A gimbal mounting may be used in place of or in addition to thespring 78. The spring or gimbal mounting thereby allows for a limitedamount of misalignment between stage 10 and members 40C, 40D (not shownin FIG. 3A).

[0037]FIG. 4 is a top view of a photolithography apparatus including thestage mechanism of FIGS. 1 and 2 and further including, in addition tothe elements shown in FIG. 1, the supporting base structure 100 whichsupports the photolithography apparatus including frame 94 except forthe reticle stage mechanism. (Not all the structures shown in FIG. 1 arelabelled in FIG. 4, for simplicity.) Base structure 100 supports fourvertical support pillars 102A, 102B, 102C and 102D connected tostructure 94 by respectively bracket structures 106A, 106B, 106C and106D. It is to be appreciated that the size of the base structure 100 isfairly large, i.e. approximately 3 meters top to bottom in oneembodiment. Each pillar 102A, 102B, 102C, 102D includes an internalconventional servo mechanism (not shown) for leveling purposes. Alsoshown in FIG. 4 are the supports 108 and 110 for respectively laserinterferometer units (beam splitter etc.) 112A, 112B, 112C. FIG. 4 willbe further understood with reference to FIG. 5 which shows a view ofFIG. 4 through cross-sectional line 5-5 of FIG. 4.

[0038] In FIGS. 4 and 5 the full extent of the supporting structure 94can be seen along with its support pillars 102A, 102C which rest on thebase structure 100 which is in contact with the ground via aconventional foundation (not shown). The independent support structurefor the reticle stage base support structure 80 is shown, in FIG. 4 only(for clarity) and similarly includes a set of four pillars 114A, 114B,114C, 114D with associated bracket structures 116A, 116B, 116C, 116D,with the pillars thereby extending from the level of base supportstructure 80 down to the base structure 100.

[0039] The lower portion of FIG. 5 shows the wafer stage 120 andassociated support structures 122, 124. The elements of wafer stage 120conventionally include (not labelled in the drawing) a base, the stageitself, fixed stage guides located on the base, magnetic tracks locatedon the fixed stage guides, and motor coils fitting in the magnetictracks and connected to the stage itself. Laser beams from laser 124mounted on support 126 locate lens 92 and the stage itself byinterferometry.

[0040]FIG. 6A shows detail of one of the window frame guide hingedflexure structures, e.g. 44C, in a top view (corresponding to FIG. 1).Each of hinges 44A, 44B, 44C and 44D is identical. These flexure hingeshave the advantage over a mechanical-type hinge of not needinglubrication, not exhibiting histeresis (as long as the flexure is notbent beyond its mechanical tolerance) and not having any mechanical“slop”, as well as being inexpensive to fabricate.

[0041] Each individual flexure is e.g. ¼ hard 302 stainless steelapproximately 20 mils (0.02 inch) thick and can sustain a maximum bendof 0.5 degree. The width of each flexure is not critical; a typicalwidth is 0.5 inch. Two, three or four flexures are used at each hinge44A, 44B, 44C and 44D in FIG. 1. The number of flexures used at eachhinge is essentially determined by the amount of space available, i.e.,the height of the window frame guide members. The four individualflexures 130A, 130B, 130C, 130D shown in FIG. 6A (and also in a 90°rotated view in FIG. 6B) are each attached by clamps 136A, 136B, 136C,136D to adjacent frame members (members 40B and 40D in FIGS. 6A and 6B)by conventional screws which pass through holes in the individualflexures 130A, 130B, 130C, 130D and through the clamps and are securedin corresponding threaded holes in frame members 40B and 40D.

[0042] Note that the frame members 40B, 40D of FIGS. 6A and 6B differsomewhat from those of FIG. 1 in terms of the angular (triangular)structures at the ends of frame members 40B, 40D and to which the metalflexures 130A, 130B, 130C, 130D are mounted. In the embodiment of FIG.1, these angular structures are dispensed with, although their presencemakes screw mounting of the flexures easier.

[0043] In an alternate embodiment, the window frame guide is not hingedbut is a rigid structure. To accommodate this rigidity and preventbinding, one of bearings 72C or 72B is eliminated, and the remainingbearing moved to the center of member 40B, mounted on a gimbal with nospring. The other bearings (except those mounted on stage 10) are alsogimballed.

[0044] This disclosure is illustrative and not limiting; furthermodifications will be apparent to one skilled in the art in light ofthis disclosure and are intended to fall within the scope of theappended claims.

I claim:
 1. A stage mechanism capable of precision motion, comprising: abase defining a principal surface; a movable stage located on theprincipal surface of the base; a frame of four members connectedtogether and laterally surrounding the stage, a first and second of themembers being opposing and in slidable contact with the stage; and firstand second fixed guides mounted spaced apart and parallel to each other,a third and fourth of the members being opposing and in slidable contactwith respectively the first and second fixed guides.
 2. The stagemechanism of claim 1 , wherein the first and second members are eachconnected to both the third and fourth members by flexures, therebyallowing movement of each member relative to the other members.
 3. Thestage mechanism of claim 2 , wherein each flexure comprises at least twostrips of metal, the two strips each being connected to two of themembers, and the two strips overlying one another in an X configuration.4. The stage mechanism of claim 1 , further comprising a plurality offluid bearings located on an underside of the stage and confronting thebase.
 5. The stage mechanism of claim 1 , further comprising at leasttwo fluid bearings located on each of the third and fourth members andconfronting, respectively, surfaces of the first and second fixedguides.
 6. The stage mechanism of claim 1 , further comprising a firstmotor element on each of the third and fourth members, thereby drivingthe frame relative to the first and second fixed guides.
 7. The stagemechanism of claim 6 , further comprising a second motor element on thebase and associated with each of the respectively first and second fixedguides co-operating with one of the first motor elements.
 8. The stagemechanism of claim 1 , further comprising a first motor element locatedon opposing sides of the stage confronting respectively the first andsecond members, thereby driving the stage relative to the first andsecond members.
 9. The stage mechanism of claim 1 , further comprising afluid bearing spring mounted on one of the third and fourth members andconfronting a surface of one of the first and second fixed guides,thereby loading the frame against the first and second fixed guides. 10.The stage mechanism of claim 1 , further comprising a support base forthe first and second fixed guides and the frame members, the supportbase being supported independently of the base on which the stage islocated.
 11. A method of operating a stage capable of precision movementin two perpendicular directions, comprising the steps of: locating thestage on a base; laterally surrounding the stage with a frame inslidable contact with the stage; driving the frame in a first of the twoperpendicular directions relative to the base; and driving the stage ina second of the two perpendicular directions relative to the frame. 12.The method of claim 11 , the frame being adapted for flexing in the twoperpendicular directions.
 13. The method of claim 11 , furthercomprising supporting the base independently from the frame.
 14. Ahinged frame structure, capable of deformation, comprising: four rigidframe members arranged in a rectangle, each frame member near each ofits ends defining an angular structure lying adjacent a similar angularstructure of an adjacent frame member; and at least two metal flexuresconnecting each two adjacent angular structures, wherein the two metalflexures cross in an X configuration.
 15. The hinged frame structure ofclaim 14 , wherein each metal flexure is of stainless steel less than0.05 inches thick.